Ultrafast time-evolution of chiral N\'eel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials

Dynamique de la chiralité dans des multicouches observées par diffusion magnétique résonante des rayons X

Chirality and topology have recently emerged in nanomagnetism, both for studying new magnetic textures with alluring properties such as chiral domain walls (DW) or magnetic skyrmions, and for looking at new routes towards future disruptive spintronic devices. Besides the race towards ultimate size magnetic bits, revealing how the magnetization can be altered at the ultimate timescale is another key question to address the actual challenges of future data storage, spin logic or even neuromorphic spintronic technologies.In this PhD work, the study of different system ranging from Synthetic Antiferromagnets (SAF) and chiral skyrmions or DW in ferromagnetic multilayers (FM). has been performed by X-ray Resonant Magnetic Scattering (XRMS).First, static experiment have been performed and the XRMS techniques description on FM is followed by the demonstration that this approach can also be used on SAF samples that allows higher displacement velocities of magnetic textures than FM. SAFs are made of FM layers coupled antiferromagnetically to each other and are difficult to study with conventional techniques due to the zero net magnetization in the multilayer, this PhD demonstrates that XRMS is one of the few very sensitive technique to probe the chirality in such system.Secondly, time-resolved study on FM with chiral DWs is performed in pump and probe mode at FERMI free electron laser. We found that the DWs demagnetizes more and recovers faster than the domains. A hot electron induced torque model has been proposed to explain this time evolution.Finally, preliminary time-resolved experiments on a skyrmions lattice state in FM has been also performed. We found no significant difference between DW and skyrmions in the ultra-fast timescale (30 ps), a significant change with field is observed. The understanding of this new result is being discussed but is still not yet complete and deserve future study.Les notions de chiralité et de topologie des textures magnétiques ont récemment émergé en nanomagnétisme dans le cadre de l'étude des parois de domaines chirales ou des skyrmions dans une perspective de nouvelles technologies de stockage de l'information. Au-delà de la course vers la réduction de la taille des bits magnétiques, l'accélération de la dynamique de l'aimantation est un autre défi à surmonter pour de futurs application de l'électronique de spin, la spintronique.Dans ce travail de doctorat, l'étude de différents systèmes allant des échantillons antiferromagnétiques synthétiques (SAF) aux skyrmions et parois de domaine chiraux dans des multicouches ferromagnétiques (FM) a été réalisée par diffraction résonante magnétique des rayons X (XRMS). Dans un premier temps, les résultats expérimentaux non dépendants du temps réalisés sur des échantillons FM par XRMS sont décrits. Cette partie est suivie par la démonstration que la XRMS est possible sur des échantillons SAFs. Les SAFs sont constitués de couches FM couplées antiferromagnétiquement et bien qu'ils permettent d'atteindre des vitesses de déplacement de texture magnétiques supérieures au FM, leur caractérisation avec des techniques conventionnelles est difficile due à l'aimantation totale nulle dans la multicouche. Ce travail doctorale montre que la XRMS est une des rares techniques très sensible pour sonder la chiralité dans ces systèmes.Dans un deuxième temps, l'étude résolue en temps sur des échantillons FM ayant des parois de domaines chirales, est effectuée en mode pompe et sonde au laser à électrons libres de FERMI. Nous avons trouvé que les parois de domaines se désaimantent plus et se ré-aimantent plus vite que les domaines. Un modèle fondé sur l'action d'un couple induit par les électrons chauds polarisés en spin et venant des domaines vers les parois a été proposé pour expliquer cette évolution temporelle.Finalement, une étude préliminaire résolue en temps sur des échantillons FM présentant un réseau de skyrmion sous champ magnétique a été réalisée. Nous n'avons trouvé aucune différence significative entre l'état domaine et l'état skyrmionique dans les dix première picosecondes. Cependant, après trente picosecondes, un changement significatif de comportement est observé en champ. La compréhension de ce nouveau résultat est discutée mais n'est pas complète, et mérite des études complémentaires

General treatment of off-specular resonant soft x-ray magnetic scattering using the distorted-wave Born approximation: Numerical algorithm and experimental studies with hybrid chiral domain structures

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Chiral spin spiral in synthetic antiferromagnets probed by circular dichroism in x-ray resonant magnetic scattering

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Polar Chirality in BiFeO 3 Emerging from A Peculiar Domain Wall Sequence

International audienceTopological states are currently gathering intensive investigation in condensed matter physics due to their potential as configurable electronic devices for the future era coined "topotronics". Beyond numerous breakthroughs in magnetism over the last decade, a new paradigm is emerging with the proposal of topologically-protected objects in ferroelectric materials. Recently, ferroelectric skyrmions and vortices were observed in PbTiO3/SrTiO3 superlattices, opening the path towards ultra-small topological objects with low-power electric-field control. Here we report the observation of chiral polar windings in a single epitaxial thin film, triggered by its self-organized stripe domain pattern arrangement. Combining resonant elastic X-ray scattering and scanning transmission electron microscopy, we show signatures of polar chirality in epitaxial BiFeO3 thin films corroborated with a complex ferroelectric domain wall structure. The net chirality suggests that domain walls induce a polar rotation through a small path alternating with an unexpected long path at every second domain wall. In addition, scanning probe microscopy reveals singularities associated to this peculiar domain wall structure. These results bring new insights into the unexpected complexity of standard striped-domain BiFeO3 thin films and open questions as for the driving force of this polar chirality

Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm (≃310 eV), we were able to probe close to the Fe L3 edge (706.8 eV) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization

Isotope ratios of H, C, and O in CO2 and H2O of the Martian atmosphere

Stable isotope ratios of H, C, and O are powerful indicators of a wide variety of planetary geophysical processes, and for Mars they reveal the record of loss of its atmosphere and subsequent interactions with its surface such as carbonate formation. We report in situ measurements of the isotopic ratios of D/H and O-18/O-16 in water and C-13/C-12, O-18/O-16, O-17/O-16, and (CO)-C-13-O-18/(CO)-C-12-O-16 in carbon dioxide, made in the martian atmosphere at Gale Crater from the Curiosity rover using the Sample Analysis at Mars (SAM)'s tunable laser spectrometer (TLS). Comparison between our measurements in the modern atmosphere and those of martian meteorites such as ALH 84001 implies that the martian reservoirs of CO2 and H2O were largely established similar to 4 billion years ago, but that atmospheric loss or surface interaction may be still ongoing